Comminution and Flotation - an intimate relationship

I would think that every mineral processor is, or should be, aware that flotation efficiency is very much linked to the product from the upstream comminution circuit. Nevertheless it is common at large conferences, such as IMPCs and SMEs, for flotation and comminution sessions to run in parallel, such that there is little opportunity for specialists in these two areas to discuss comminution-flotation as a single entity. 

I am also aware, of course, that MEI's focussed comminution and flotation conferences, by attracting specialists in these two fields, do not encourage potential fruitful collaboration. This is something that we are trying to change by inviting specialists in comminution to present appropriate work at flotation and vice-versa. It is happening only slowly however but one company that sets an example is Magotteaux, regular sponsors of MEI's comminution and flotation series, and Dr. Chris Greet, Manager Metallurgy-Minerals Processing Research at Magotteaux Australia, who has presented papers at most of the flotation conferences, and in recent years the comminution conferences, was an ideal person with whom to discuss this important link.

Chris Greet (right) at the Magotteaux booth, Flotation '13

I asked him first of all why Magotteaux obviously finds value in the MEI Conferences. He said "Magotteaux see value in both the Comminution and Flotation conferences for a number of reasons. The obvious ones are the marketing and networking opportunities, and I know our sales team do use these conferences to meet with customers (both current and potential). However, there are deeper reasons for our attendance. Both conferences provide our technical team with an opportunity to see what is new, gain a better understanding of technical problems and catch up with renowned authorities to discuss issues that we face in our technical work. The MEI conferences do bring together a very diverse group of people and it is through discussion, debate and the occasional drop of good South African red wine that we are able to focus our technical efforts to assist sales to provide the right product for their application".

He continued "Having attended both the Comminution and Flotation conferences I do note however that there does appear (at times) to be a silo mentality. At Comminution there is a huge amount of excellent work describing how breakage occurs and trying to develop more energy efficient methods to reduce particle size, but there are few studies that look beyond this to the downstream processes. Equally, at Flotation how a particle is prepared for flotation is often ignored because the exciting work is in understanding collector adsorption or the flotation behaviour of different forms of the same mineral. Do not get me wrong, the work done in both fields of endeavour is very important, but there needs to be more cross fertilisation. Finally, all is not lost, I have noticed that the topics of grinding chemistry and pulp chemistry and their influence on flotation are increasingly being discussed at conferences. This is very pleasing as I have been practicing in this area for over 25 years and I think I am starting to make some headway . . ."

Chris has highlighted here something that we have been trying to promote in recent years, and would like to become a feature of the Comminution and Flotation conferences, the fact that flotation is intimately linked to comminution, not only in regard to the size and liberation of particles, but also to pulp chemistry, and I asked him to elaborate on this:

"At plant level, most metallurgists realise that if the comminution circuit operates smoothly they have a better chance of maintaining a reasonably steady flotation circuit, and achieve the target concentrate grades and recoveries" he said. "Their primary motivation in operating the comminution circuit is to achieve a particle size distribution that has been nominated, which is presumably based on achieving liberation. Or, to put it another way the grinding circuit is designed to operate to achieve a particular percentage liberation for the subsequent flotation stage. So, in most thinking metallurgist’s mind grinding is associated with liberation.

However, there is more to it than this, as during grinding fresh particle surfaces are generated and thanks to the likes of Gaudin (or maybe his name is used in vain) it is thought that the addition of reagents (collectors, depressants, pH modifiers, activators) will have a beneficial impact on flotation. As a consequence many believe adding the reagents to grinding (either fully or partially) is a good thing. In principal it is a good plant practice to follow, but only if the chemistry is amenable.

In many instances the grinding chemistry is not taken into account. The main concerns surrounding grinding are: energy efficiency, media consumption, liner wear, and achieving the right particle size distribution. All are very important, however if the product from the comminution circuit does not produce particles with the right surface chemistry it is highly likely that concentrate grades and mineral recoveries will suffer. Despite the many non-believers grinding chemistry is critical to achieving metallurgical excellence.

There are many articles in the literature that talk of the necessity of operating in the right Eh regime to achieve collector adsorption. The CSIRO have published extensively in this area, and have shown that most sulphide minerals have an Eh threshold below which collector adsorption is weak and recoveries are low. In most instances the Eh of the plant mill discharge is well below this threshold, yet plants continue to add collector during grinding, thinking that they are doing the right thing.

For example, the threshold Eh for collector adsorption onto pentlandite is about +187 mV (SHE), and the mill discharge Eh can be as low as -250 mV (SHE). So adding the collector to the mill does not enhance pentlandite flotation. Yes, once the pulp arrives in the flotation cell and is aerated the Eh will increase and collector adsorption will occur. However, the likelihood is that the collector would have interacted with other minerals within the mill (for example, chalcopyrite, pyrrhotite), which have a lower threshold, and what results is a very non-selective flotation.

There are other reactions that occur within the comminution circuit which influence what happens in the flotation circuit. The potential list of reactions (in no particular order) are:

• The precipitation of ionic species from the process water;
• Inadvertent activation or depression of mineral species by ionic species and reagents in the process water;
• The dissolution of minerals (i.e. dolomite) and the deposition of these ions onto surfaces;
• The interaction of reagents with each other, mineral surfaces and grinding media;
• The impact of pH (e.g. below pH 10.5 Mg(OH)2 is soluble in water above this pH is precipitates on the surfaces of minerals and can depress flotation);
• Mineral/mineral interactions; and
• Mineral/grinding media interactions.

All impact the pulp and surface chemistry and have an influence on the outcomes of the flotation process. So, an understanding that the comminution circuit is not only about reducing the particle size to achieve adequate liberation for the subsequent flotation stage, but is also a chemical reactor where the surface chemistry of the minerals we are trying to separate is generated is a must. To maximise metallurgical outcomes the flotation operator must take an active interest in the comminution circuit".

So important is pulp chemistry that Magotteaux has patented a laboratory mill designed to reproduce the pulp chemical conditions of the plant grinding mill in the laboratory. The Magotteaux Mill® allows the user to investigate the impact of grinding chemistry on their metallurgical process. The product generated in the laboratory has nominally the same physical properties (particle size distribution) and pulp chemical properties (Eh, pH, dissolved oxygen, oxygen demand and EDTA extractable iron) as an equivalent sample taken from the plant. This signifies a distinct advantage in testing as the results produced in the laboratory are more representative of those that would be observed in a plant.

The UK company Grinding Solutions Ltd, sponsors of three MEI Comminution conferences, including the forthcoming Comminution '18, are also very aware of the link to flotation, and recently took possession of the Magotteaux Mill in their rapidly expanding premises in Cornwall. Not surprisingly GSL's flotation consultant, Dr. Kathryn Hadler, is a regular presenter at the Flotation series of conferences and she says "The Magotteaux mill enhances significantly the ability to monitor chemical conditions during milling, which allows greater understanding (and improvement!) of flotation performance response. Furthermore, it provides an opportunity to improve the integration of mineral processing laboratory testing which is of increasing importance as we move forward with lower grade and more complex ores".

GSL staff with the new Magotteaux Mill:  From left to right:  Jon Rumbles (GSL),
Grant Small (Magotteaux), Kathryn Hadler (Flotation consultant), Tom Rescorl,
James Gaydon, Nick Wilshaw and Jack Carr (all GSL)

My hope is that these two progressive companies highlight the important links, both physical and chemical, between comminution and flotation and this awareness will be taken up by researchers in this crucial area of mineral processing.

#Comminution18

#Flotation17